1. Field of the Invention
The present invention relates to an installation for pumping a liquid/gas two-phase effluent and, more specifically, to such an installation for pumping hydrocarbons from an oil well.
2. Description of Related Art
In some oil wells, the natural flow of hydrocarbons from the bottom to the surface is not sufficient to allow or to sustain commercial production. This is due either to the viscosity and to the weight of the effluents, or to a natural pressure at the bottom of the well which is too low in comparison with the factors which oppose the raising of these effluents to the surface. In order to allow the well to be exploited on a commercial scale it is advisable to use a system for artificially raising the effluent, or well-activation system. For example, a pump may be mounted at the lower end of a production tube located in the well, or an installation for injecting gas into the bottom of the well may be provided. The latter type of installation, more commonly known as a gas lift, is used to lighten the column of hydrocarbons located in the well in order to make it easier to raise to the surface.
An installation for injecting gas into the bottom of a well is generally reliable, but has the drawback of requiring, on an isolated site, a source of pressurized gas, for example a compressor and its associated pipe-work.
The use of a pump, placed at the lower end of a tubing via which the liquid/gas two-phase effluent is raised to the surface, has drawbacks when this effluent contains a significant proportion of gas. The bubbles contained in the effluent are compressible, a fraction of the pump energy being used to compress the gas rather than to convey the fluid to the surface. This phenomenon may even lead to the flow rate of pumped fluid becoming zero (a situation commonly known as cavitation or gas lock). Centrifugal pumps are particularly susceptible to gas lock, particularly in wells because they are situated at the foot of a column of fluid which, on account of its own weight, creates a hydrostatic back pressure which, even at zero flow rate, opposes delivery. What is more, during flow stoppages, the gases and liquids end up separating under gravity at the bottom of the well and this, under certain circumstances, creates severe malfunctioning of the pump when it restarts if the accumulated gas enters the pump or even if, under these transient conditions, a large bubble of gas has been formed inside the pump.
It is therefore advisable for most of the gas to be separated from the liquid phase of the effluent before this liquid is drawn in by the pump. Thus, all the pump energy can be expended in conveying the liquid to the surface, and the risks of cavitation are reduced.
However, this separating of gas upstream of the pump requires a gas discharge pipe which differs from the one used by the liquid passing through the pump. A common way of fulfilling this function is to let the gas "ventilate"--that is to say travel--through the annular space there is between the internal wall of the well casing and the external wall of the tubing used for the flow of the pumped liquid. This method does, however, present a number of major drawbacks, the consequence of which is that of making the exploitation of the well more expensive and even dangerous: in particular the loss of natural raising energy; the chemical and/or mechanical attack of the materials in contact with the gas; and significant and uncontrollable exchanges of heat between the effluents and the perimeter of the well, which may give rise to expensive flow problems.
In order partially to alleviate these drawbacks, the document FR-A-2,723,143 describes an installation for an oil well comprising a pump placed at the lower end of a first tubing, a second tubing being intended to receive, as necessary, gas from the effluent and separated upstream of the pump, and to convey it as far as the surface independently of the liquid phase. In this document, in order to encourage the separation of the gas from the effluent at the bottom of the well, the pump has a sleeve which extends as far as a level below the layer of oil-bearing rock. Thus, the effluent entering the well is forced to flow downwards before being drawn up by the pump, and this has the effect of guaranteeing excellent separation of the gas intended to take the independent tubing.
The installation described in document FR-A-2,723,143, although allowing the pump to receive an effluent that contains a low gas content, does, however, have drawbacks in that it requires a second tubing along the entire length of the well, something which results in substantial dimensional and economic constraints in the work. Furthermore, the column of liquid effluent raised to the surface by the pump is heavy, because it is essentially free of gas, and this means that a greater pumping power is required.